answer:Honestly, there’s probably no other reason than because that was the standard at the time. In 2006 (which is when the site launched), basically every website used a username to login. Even now, the vast majority work that way. It’s definitely easier to break into an account when you start with half of the needed information, but it also used to be a lot easier to lose access to an email address. Maybe that had something to do with the username becoming such a ubiquitous login item? (I’m no web designer either, so I can’t say for sure.)